What is the difference between Cooling and Heating Degree Days?

While Cooling Degree Days (CDD) represent a value that can be used for analyzing the demand for cooling of a building, Heating Degree Days (HDD) are used for analyses and optimization of heating-related energy usage. 

To calculate CDDs, you sum up the difference between the outside and cooling base temperature (also called the cooling balance point). That is the outside air temperature above which the building needs cooling. Days where the outside temperature is not exceeding the base temperature, are counted as CDD=0. 

The calculation of HDDs, on the other hand, sums up the difference between the heating base temperature (also called the heating balance point)  and the outside temperature. The heating base temperature is the outside air temperature below which the building needs heating.

Days when the outside temperature drops below the base temperature, are counted as HDD=0.